Display panel and method of operating said panel to produce different colors of light output

ABSTRACT

A display panel includes a plurality of cells containing a phosphor material sensitive to ultraviolet light and an ionizable gas which has a characteristic color of cathode glow and, also, generates ultraviolet light when discharged. The gas filling also includes mercury vapor. The method of the invention comprises operating cells at different levels of intensity, determined by input signals so that, at a low level of intensity of cell ionization and current flow, the phosphor is energized and produces its characteristic color of glow which is the dominant light output from the cell. In addition, by operating the panel at a higher level of ionization intensity and current flow, the characteristic color of cathode glow of the gas dominates the phosphor glow and provides the primary glow visible to a viewer. Similarly, if input data signals have widely varying intensities, then the current flow through the display cells varies similarly and the color of light output from the cells varies over a wide range, depending on the relative contribution of each of the components in the cell, the phosphor, the gas, and the mercury.

United States Patent Cola [ DISPLAY PANEL AND METHOD OF OPERATING SAID PANEL TO PRODUCE DIFFERENT COLORS OF LIGHT OUTPUT [72] Inventor: Rudolph A. Cola, Malvern, Pa.

[73] Assignee: Burroughs Corporation,

Mich.

221 Filed: March 19, 1971 21 App1.No.: 126,154

Detroit,

52 us. c1 ..3l3/108 R, 313/188, 315/169,

313/108 B 51 1m. (:1 ..H01j 11/02 [58] Field ofSearch. ..313 /108,108 A, 108 B, 109.5,

[ 1 Nov. 28, 1972 Primary Examinerl-lerman Karl Saalbach Assistant Examiner-Saxfield Chatmon, Jr. Attorney-Kenneth L. Miller, Robert A. Green, George L. Kensinger and Charles S. Hall [57] ABSTRACT A display panel includes a plurality of cells containing a phosphor material sensitive to ultraviolet light and an ionizable gas which has a characteristic color of cathode glow and, also, generates ultraviolet light when discharged. The gas filling also includes mercury vapor. The method of the invention comprises operating cells at different levels of intensity, determined by input signals so that, at a low level of intensity of cell ionization and current flow, the phosphor is energized and produces its characteristic color of glow which is the dominant light output from the cell. In addition, by operating the panel at a higher level of ionization intensity and current flow, the characteristic color of cathode glow of the gas dominates the phosphor glow and provides the primary glow visible to a viewer.

Similarly, if input data signals have widely varying intensities, then the current flow through the display cells varies similarly and the color of light output from the cells varies over a wide range, depending on the relative contribution of each of the components in the cell, the phosphor, the gas, and the mercury. I 12 Claiim, 8 Drawing figures PATENTED NOV 2 8 I972 SHEET 1 BF 5 Mumm ATTORNEY SHEET 3 OF 5 RUDOLPH COLA PATENTEU am 28 I972 PATENTED NEW 28 m2 SHEET 5 BF 5 moooo m 0000 /oooo m 0000 0000 D E R m w v,

m. 2 m m o m R A D oooo o.ooo

8 NO FILTER GR'EEN INVEN TOR. RUDOLPH COLA DISPLAY PANEL AND METHOD OF OPERATING SAID PANEL TO PRODUCE DIFFERENT COLORS OF LIGHT OUTPUT BACKGROUND OF THE INVENTION A display panel comprising a plurality of gas-filled cells which can be energized in different combinations to display messages or characters are known. Such panels generally display a message in a color which is characteristic of the gas filling of the panel. It is also known to provide phosphor materials in gas-filled display devices to provide a color of glow determined by the phosphor materials. However, the prior art provides no teaching of a display panel or method of operation thereof which provides multiple colors of glow in each of its cells.

SUMMARY OF THE INVENTION Briefly, the invention relates to a display panel having a plurality of display cells, and it comprises providing, in each display cell, a phosphor, a gas and mercury, each of which contributes to the light output from the cell in a manner controlled by the level of current flowing through the cell.

DESCRIPTION OF THE DRAWINGS FIG. 1 is an exploded perspective view of a display panel embodying the invention;

FIG. 2 is a sectional view along the lines 2--2 in FIG.

FIG. 3 is a perspective view of a modified cover plate usable in the panel of FIG. 1;

FIG. 4 is a schematic representation of a display panel embodying the invention and a circuit in which it may be operated;

FIG. 5 is a plan view of a modification of the inventron;

FIG. 6 is a sectional view, along the lines 66, in FIG. 5;

FIG. 7 is a front elevational view of a display panel of the type embodying the invention and an arrangement of light filters usable therewith; and

FIG. 8 is a front elevational view of another type of display panel embodying the invention and an arrangement of light filters usable therewith.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A display panel 10, embodying the invention, includes a bottom plate of insulating material, such as glass, ceramic, or the like, having a plurality of parallel slots 30 formed in the 'top surface thereof. Electrodes 40, which are used as scanning anodes, in one mode of operation of the panel, are seated in each of the slots 30, and electrodes 50, used as scanning cathodes, are seated on or in the top surface of plate 20. Each cathode electrode 50 crosses each anode electrode 40, 1

cells 90, also arrayed in rows and columns, with each cell being in operative relation with and positioned over a cathode aperture 70 and a scanning cell 60. A thin layer 94 of a phosphor material is provided on the walls of the cells 90. The phosphor layers 94 may be formed by a spraying process, a settling process, or the like, as is well known in the art. The phosphor 94 is preferably sensitive to ultraviolet light, and a particularly suitable phosphor is a zinc silicate phosphor GE 118-2-7 made by General Electric Co. This phosphor provides a green light output.

Display anode wires are disposed on or in the top surface of the insulating plate 80, and each is aligned with a row of display cells 90, and a glass cover plate completes the panel structure.

Preferably the lower surface of the cover plate 110 is treated to render it light-diffusing. In one suitable method of achieving this type of surface (FIG. 3), the lower surface of the plate islapped until it is a diffusing surface, and then it is coated with a thin layer 112 of insulating, preferably black, glassy material with the coating being laid down with a pattern of generally circular openings 114 corresponding to and aligned with the cells 90 in plate 80. The circular uncoated portions of the surface of the plate 110 comprise light-diffusing elements for each display cell.

The various portions of the panel 10 are secured together by means of an air-tight seal formed along the periphery thereof by means of a glass frit or the like. In addition, the panel is filled with the desired gas through a tubulation secured to the base plate 20 and communicating with the interior of the panel through a suitable hole in the base plate 20. The

panel may include any suitable gas such as neon, argon, xenon, etc., singly or in combination, with a vapor of a metal such as mercury usually included to minimize cathode sputtering. The mercury may be introduced conveniently from a capsule 154 suitably supported within the tubulation 150 and opened to release the mercury at a suitable time in the manufacturing process. This type of procedure is described in copending application Ser. No. 764,983, filed Oct. 2, 1968, now US. Pat. No. 3,580,654. The quantity of mercury introduced can be readily controlled, and, in accordance with one aspect of the invention described below, a suitably large quantity of mercury is required.

It is desirable that the selected gas be capable of generating ultraviolet light, and neon, xenon and mercury are particularly effective for this purpose. In addition, the gas used is preferably a Penning mixture in which the gases of the mixture have related energy levels such that the metastable atoms of one gas produce ions of the other gas. Neon and xenon comprise a particularly effective Penning mixture. The gas pressure at room temperature is preferably between 50 and 350 Torr or more, and, Torr has been found to be a convenient pressure.

One method of operating the display device 10 is described in detail in copending application Ser. No. 850,984, filed Aug. 18, I969. Briefly, according to this method, operating potential is applied to all of the scanning anodes 100 and sequentially to each of the scanning cathodes 50, whereby each of the columns of scanning cells 70 is fired in turn. This operation is performed cyclically and continuously through the panel and is facilitated by gas communication slots 30. Simultaneously, control signals, which may be obtained from a suitable source of information signals, are applied to eachof the anodes 100 so that at any instant as these signals are applied, the cells 90 associated with the scanning cells energized at that instant are caused to glow, and this operation if facilitated by the diffusion of excited gas particles through the appropriate cathode apertures 60. Thus, as the scanning cells are scanned and information signals are applied, a pattern of cells 90 are fired and caused to glow, and a stationary but changeable message is contained therein.

The principles underlying the operation of the invention are as follows: When a display cell 90 is turned on by an information signal, the magnitude of the current which flows through it determines the nature of its light output. In general, the magnitude of the current controls the brightness, and in this case the color, of the glow in a display cell. In panel 10, if a cell 90 is operated at a relatively low level, when the gas filling ionizes, ultraviolet light which is generated energizes the phosphor and it provides a green light output which masks this low level of glow of the gas. If the cell is operated at some higher current level, the characteristic color of glow of the gas, in this case reddishorange, predominates and masks the green glow of the phosphor. A two-color system is thus provided. In addition, at some still higher level of intensity, the mercury in the cell is ionized and emits its characteristic generally blue glow which dominates and masks all other glow. Thus a three-color system is provided and, at appropriate levels of operation, intermediate colors can be produced with suitable blending of each of the individual colors.

In actuality, every increment or level of current which flows through a display cell will produce a different color of light output, although the various color changes may be minimal from step to step and. may not be directly detectable by the human eye.

(Iircuits for operating a display panel such as panel 10, and the mode of operation thereof, are described and claimed in copending application Ser. No. 850,984, filed Aug. I8, 1969. A schematic representation of panel 10 and a portion of a circuit in which it may be operated are shown in FIG. 4. r

The circuit includes a source of data signals which may include a computer, decoders, encoders, amplifier, and the like, coupled to a character generator 130 which has an output lead 140 connected to each of the display anodes 100 to panel 10. The character generator 130 provides signals of an amplitude determined by information received from the data signal circuitry 120. The cathode electrodes 50 are connected in groups generally as shown, and each group is connected to its own source 150 of generally negative operating potential. The scanning anodes are connected to positive power supply V1 through suitable resistive paths (not shown). Suitable clock synchronizing circuits 160 are provided, coupled to the various portions of the circuit and the cathode drivers 150 so that, as each column of scanning cells is fired, the appropriate display cell is also fired and at the current level determined by the output of the character generator. It can be seen that the magnitude of the current passed through each display cell as it is tired determines the color of glow produced by that cell in accordance with the principles set forth above.

The principles of the invention may be utilized in a panel 10', also described in application Ser. No. 850,984, which has a single array of scanning cells, with the scanning anodes and cathodes reversed in position from the arrangement shown in panel 10. This form of the invention is illustrated in FIGS. 5 and 6 in panel 10 and includes an insulating base plate 200, scanning cathodes 50, which need not be apertured, seated on or in the top surface of plate 200, apertured insulating plate having cells with phosphor layers 94, and scanning anode electrodes seated on or in the top surface of the plate 80 or in the bottom surface of cover plate and oriented orthogonally to the cathode electrodes 50. In this embodiment of the invention, gas communication slots 210 are provided in plate 80 between adjacent columns of cells 90.

Panel 10 operates generally similarly to panel 10; however, only a single layer of cells 90 is present. The columns of cells 90 are scanned by applying operating potential to all of the anode electrodes 1 l0 and to each of the cathodes 50 in turn. The potentials are such that all of the cells are ionized at least to a minimum level and with minimum current which provides light output from the phosphor in each cell. This represents background glow. At the same time, certain display anodes 110 will receive information signals as determined by the data input which will provide a higher level of energization and greater current flow, and this will cause the gas glow to overcome the phosphor glow. The total number of cells thus energized display a message in the characteristic gas color against a background of phosphor glow in the other cells which do not form a part of the message.

it is to be noted that, when display panels 10 or 10 are used to display a message, in general, the display cells are operated at two distinct current levels, one being the background level which is a low current level and represents the lack of display information, and a secondlevel, higher current level, which represents the presence of display information. According to the invention, the background level is in one color and the message is in another color. If a panel is used to display a multi-color message or picture, then the display information signals applied to the display anodes would have appropriately varying current levels. If desired, the message could be embodied in what have been described as the background color.

It is clear that the principles of the invention described above with respect to two specific types of panels are also applicable to other types of display panels or display devices,

The utility of display devices such as panels 10 and 10' can be enhanced by coupling filters to the panels to take advantage of the various constituents in the output of a cell 90. An ionized cell 90 has a green color component due to the phosphor, a red color component due to the gas, and a generally bluish color component due to the mercury; and, a green filter can produce green light output, a dark red filter can produce red output, and an orange filter can produce a reddishorange light output. This, it is possible to secure one filter to a panel to obtain a selected light output, or different filters could be applied to different portions of a panel, as illustrated in FIG. 7, to obtain a different color of light output from each, with perhaps one portion having no filter at all to provide still another color ofliquid output. I

In addition, in a display panel 214 having a plurality of registers or rows of-characterrdisplaying areas 220 (FIG. 8), such panels being described and claimed in copending application Ser. No. 21,125, filed Mar. 19, 1970, each register can carry either no filter, one filter so that each register can provide its display in a characteristic color, or each register could have a plurality of filters as illustrated in FIG. 7.

What is claimed is:

l. A display panel comprising a gas-filled envelope containing a plurality of display cells arrayed in rows and columns in a common plane,

said cells including a quantity of phosphor material sensitive to ultraviolet light and a gas filling which generates ultraviolet light when ionized,

at least two electrodes associated with each of said cells for ionizing said cells at selected levels of intensity and current flow, at one of which the phosphor material in the cells is energized by the ionized gas and its light output is the primary visibleoutput from said cells, at another of which the gas generates its characteristic color of cathode glow light which. masks the glow of said phosphor material, and

means coupled to said electrodes for ionizing a plurality of said cells and causing selected levels of current to flow therethrough to produce different selected colors of light output from said panel.

2. The panel defined in claim 1 wherein said lastnamed means maintains some of said cells at a low level of currentflow to provide background color and others of said cells at a higher level of current fiow to provide a visible display of different color against said background color.

3. The panel defined in claim 1 and including mercury vapor in said gas filling whereby said mercury vapor affects the color of glow in said gasdepending on the level of current flow in a cell.

4. A display panel comprising a gas-filled envelope including a plurality of display cells arrayed in rows and columns in a common layer, a

' a plurality of cathode electrodes in operative relation with a portion of each of said display cells and each cathode being aligned with a column of said cells,

a plurality of anode electrodes in operative relation with another portion of each of said cells, each anode electrode being aligned with a row of said cells and oriented orthogonally to each of said cathode electrodes,

. means coupled to said anode and cathode electrodes a phosphor material in each of said display cells, said component to the cathode glow produced in each cell, and

associated with each of said cells for ionizing said cells at selected levels of intensity and current flow, at one of which the phosphor material in the cells is energized and its light output is the primary visible output from said cells, at another of which the gas generates its characteristic color of cathode glow light which masks the glow of said phosphor material, and said circuit means being adapted to ionize a'plurality of said cells simultaneously to at least one of said levels of intensity.

5. A display panel comprising a gas-filled envelope including an insulating plate having top and bottom surfaces and including a plurality of display cells arrayed in rows and columns in a common plane,

a plurality of cathode electrodes in operative relation with the bottom surface of said plate, each cathode being aligned with acolumn of said cells,

a plurality of anode electrodes in operative relation with the top surface of said plate, each anode electrode being aligned with a row of said' cells and oriented orthogonally to each of said cathode electrodes,

a phosphor material in each of said display cells, said phosphor material being adapted to generate a characteristic color of light when energized by the excited gas filling in said envelope,

said gas having a characteristic color of light emission when energized,

' mercury vapor in said gas for providing another color component to the cathode glow produced in each cell, and

circuit means coupled to said cathode electrodes and said anode electrodes for ionizing said cells at selected levels of intensity and current flow, at one of which'the phosphor materialin the cells is energized and its light output is the primary visible output from said cells, at another of which the gas generates its characteristic color of cathode glow light which masks the glow of said phosphor material, said circuit means also serving to ionize a plurality of said cells simultaneously to at least one of said levels of intensity and current flow.

)6. A display panel comprising a gas-filled envelope,

an array of first cells disposed in rows and columns in a common plane,

an array of second cells disposed in rows and columns in a common plane,

said second cells being disposed vertically above said first cells with each second cell communicating with a first cell,

an array of scanning anodes associated with said first cells, with each anode being aligned with a row of said first cells,

a plurality of scanning cathode electrodes disposed between said arrays of said first and second cells, each scanning cathode being aligned with a column of said second cells,

a plurality of display anodes disposed in operative relation with said second cells, each display anode being aligned with a row of said second cells,

a phosphor material disposed on the wall of each of said second cells, said phosphor material being sensitive to radiation generated by said gas when it is ionized,

mercury vapor in the gas filling in said envelope, and

circuit means coupled to said cathode electrodes and said display anodes for ionizing said second cells at selected levels of intensity and current flow, at one of whichthe phosphor material in the cells is energized and its light output is the primary visible output from said second cells, at another of which-the gas generates its characteristic color of cathode glow light which masks the glow of said phosphor material, said circuit means also serving to ionize a plurality of said second cells simultaneously to at least one of said levels of intensity.

7. A display'panel comprising a gas-filled envelope, I

a first insulating plate in said envelope having a top surface and having a plurality of parallel horizontal slots formed in said top surface, scanning anode electrode seated in each of said slots, 1 v a 1 a plurality of apertured scanning cathode electrodes disposed adjacent to the top surface of said first plate and spaced from said scanning anodes and oriented orthogonally thereto so that each cathode crosses each anode and each crossing defines a scanning cell, said scanning cell being arrayed in rows and columns,

r, a second plate having top and bottom surfaces and a with said cathodes between the two plates and with i said display cells being disposed vertically above said scanning cells with each display cell communicating with a scanning cell through an aperture in a cathode,

a plurality of display anodes disposed on or in the top surface of said second plate, each display anode being aligned with a row of said display cells,

a phosphor material disposed on the wall of each of said display cells, said phosphor material being sensitive to radiation generated by said gas when it is ionized,

mercury vapor in the gas filling in said envelope, and

circuit means coupled to said cathode electrodes and said scanning anodes for ionizing said display cells at selected levels of intensity and current flow, at one of which the phosphor material in the cells is energized and its light output is the primary visible output from said display cells, at another of which the gas generates its characteristic color of cathode glow light which masks the glow of said ble levels of current flow through said display cells each level of current flow producing a different color of light output therein.

10. A display panel comprising a gas-filled envelope containing aplurality of display cells selectively energizable todisplay a message, electrodes coupled to said cells for selectively ioniz- 7 ing and firing said cells, j said cells including phosphor material sensitive to ultraviolet light, a gas filling which generates ultraviolet light and visible light whenionized, and

mercury vapor,

each of said constituents of said cells contributing to the light output therefrom in different degrees at different levels of current flow, and

circuit means coupled to said electrodes for ionizing said cells at selected levels of intensity and current flow, at one of which the phosphor material in the cells is energized and its lightoutput is the primary visible output from said second cells, at another of which the gas generates its characteristic color of cathode glow light which masks the glow of said phosphor material, said circuit means also serving to ionize a plurality of said cells simultaneously to at least one of said levels of intensity.

11 The panel defined in claim 10 and at least one light filter coupled to said cells for selecting a component of the light generated therein.

12. A method of operating a display panel made up of a plurality of display cells each containing a lightemitting phosphor material, comprising the steps of operating all of said cells at afirst relatively low level of ionization and current flow such that said phosphor material is energized and all of said cells provide light output having the color characteristic of said phosphor, and I operating others of said cells at another level of ionization and current flow whereby the gas in said cells generates its characteristic light output at such a level that it masks the light output of said phosphor material. 

1. A display panel comprising a gas-filled envelope containing a plurality of display cells arrayed in rows and columns in a common plane, said cells including a quantity of phosphor material sensitive to ultravioLet light and a gas filling which generates ultraviolet light when ionized, at least two electrodes associated with each of said cells for ionizing said cells at selected levels of intensity and current flow, at one of which the phosphor material in the cells is energized by the ionized gas and its light output is the primary visible output from said cells, at another of which the gas generates its characteristic color of cathode glow light which masks the glow of said phosphor material, and means coupled to said electrodes for ionizing a plurality of said cells and causing selected levels of current to flow therethrough to produce different selected colors of light output from said panel.
 2. The panel defined in claim 1 wherein said last-named means maintains some of said cells at a low level of current flow to provide background color and others of said cells at a higher level of current flow to provide a visible display of different color against said background color.
 3. The panel defined in claim 1 and including mercury vapor in said gas filling whereby said mercury vapor affects the color of glow in said gas depending on the level of current flow in a cell.
 4. A display panel comprising a gas-filled envelope including a plurality of display cells arrayed in rows and columns in a common layer, a plurality of cathode electrodes in operative relation with a portion of each of said display cells and each cathode being aligned with a column of said cells, a plurality of anode electrodes in operative relation with another portion of each of said cells, each anode electrode being aligned with a row of said cells and oriented orthogonally to each of said cathode electrodes, a phosphor material in each of said display cells, said phosphor material being adapted to generated a characteristic color of light when energized by the excited gas filling in said envelope, said gas having a characteristic color of light emission when energized, mercury vapor in said gas for providing another color component to the cathode glow produced in each cell, and means coupled to said anode and cathode electrodes associated with each of said cells for ionizing said cells at selected levels of intensity and current flow, at one of which the phosphor material in the cells is energized and its light output is the primary visible output from said cells, at another of which the gas generates its characteristic color of cathode glow light which masks the glow of said phosphor material, and said circuit means being adapted to ionize a plurality of said cells simultaneously to at least one of said levels of intensity.
 5. A display panel comprising a gas-filled envelope including an insulating plate having top and bottom surfaces and including a plurality of display cells arrayed in rows and columns in a common plane, a plurality of cathode electrodes in operative relation with the bottom surface of said plate, each cathode being aligned with a column of said cells, a plurality of anode electrodes in operative relation with the top surface of said plate, each anode electrode being aligned with a row of said cells and oriented orthogonally to each of said cathode electrodes, a phosphor material in each of said display cells, said phosphor material being adapted to generate a characteristic color of light when energized by the excited gas filling in said envelope, said gas having a characteristic color of light emission when energized, mercury vapor in said gas for providing another color component to the cathode glow produced in each cell, and circuit means coupled to said cathode electrodes and said anode electrodes for ionizing said cells at selected levels of intensity and current flow, at one of which the phosphor material in the cells is energized and its light output is the primary visible output from said cells, at another of which the gas generates its characteristic color of cathode glow light which masks tHe glow of said phosphor material, said circuit means also serving to ionize a plurality of said cells simultaneously to at least one of said levels of intensity and current flow.
 6. A display panel comprising a gas-filled envelope, an array of first cells disposed in rows and columns in a common plane, an array of second cells disposed in rows and columns in a common plane, said second cells being disposed vertically above said first cells with each second cell communicating with a first cell, an array of scanning anodes associated with said first cells, with each anode being aligned with a row of said first cells, a plurality of scanning cathode electrodes disposed between said arrays of said first and second cells, each scanning cathode being aligned with a column of said second cells, a plurality of display anodes disposed in operative relation with said second cells, each display anode being aligned with a row of said second cells, a phosphor material disposed on the wall of each of said second cells, said phosphor material being sensitive to radiation generated by said gas when it is ionized, mercury vapor in the gas filling in said envelope, and circuit means coupled to said cathode electrodes and said display anodes for ionizing said second cells at selected levels of intensity and current flow, at one of which the phosphor material in the cells is energized and its light output is the primary visible output from said second cells, at another of which the gas generates its characteristic color of cathode glow light which masks the glow of said phosphor material, said circuit means also serving to ionize a plurality of said second cells simultaneously to at least one of said levels of intensity.
 7. A display panel comprising a gas-filled envelope, a first insulating plate in said envelope having a top surface and having a plurality of parallel horizontal slots formed in said top surface, a scanning anode electrode seated in each of said slots, a plurality of apertured scanning cathode electrodes disposed adjacent to the top surface of said first plate and spaced from said scanning anodes and oriented orthogonally thereto so that each cathode crosses each anode and each crossing defines a scanning cell, said scanning cell being arrayed in rows and columns, a second plate having top and bottom surfaces and a plurality of apertures comprising display cells which are arrayed in rows and columns, said second plate being positioned above said first plate with said cathodes between the two plates and with said display cells being disposed vertically above said scanning cells with each display cell communicating with a scanning cell through an aperture in a cathode, a plurality of display anodes disposed on or in the top surface of said second plate, each display anode being aligned with a row of said display cells, a phosphor material disposed on the wall of each of said display cells, said phosphor material being sensitive to radiation generated by said gas when it is ionized, mercury vapor in the gas filling in said envelope, and circuit means coupled to said cathode electrodes and said scanning anodes for ionizing said display cells at selected levels of intensity and current flow, at one of which the phosphor material in the cells is energized and its light output is the primary visible output from said display cells, at another of which the gas generates its characteristic color of cathode glow light which masks the glow of said phosphor material, said circuit means also serving to ionize a plurality of said display cells simultaneously to at least one of said levels of intensity.
 8. The panel defined in claim 7 wherein said circuit means maintains some of said display cells at a low level of intensity to provide background color and others of said display cells at a higher level of intensity to provide a visible character display against said background color.
 9. The panel defined in claim 7 wherein said circuit means can produce a wide range of continuously variable levels of current flow through said display cells, each level of current flow producing a different color of light output therein.
 10. A display panel comprising a gas-filled envelope containing a plurality of display cells selectively energizable to display a message, electrodes coupled to said cells for selectively ionizing and firing said cells, said cells including phosphor material sensitive to ultraviolet light, a gas filling which generates ultraviolet light and visible light when ionized, and mercury vapor, each of said constituents of said cells contributing to the light output therefrom in different degrees at different levels of current flow, and circuit means coupled to said electrodes for ionizing said cells at selected levels of intensity and current flow, at one of which the phosphor material in the cells is energized and its light output is the primary visible output from said second cells, at another of which the gas generates its characteristic color of cathode glow light which masks the glow of said phosphor material, said circuit means also serving to ionize a plurality of said cells simultaneously to at least one of said levels of intensity. 11 The panel defined in claim 10 and at least one light filter coupled to said cells for selecting a component of the light generated therein.
 12. A method of operating a display panel made up of a plurality of display cells each containing a light-emitting phosphor material, comprising the steps of operating all of said cells at a first relatively low level of ionization and current flow such that said phosphor material is energized and all of said cells provide light output having the color characteristic of said phosphor, and operating others of said cells at another level of ionization and current flow whereby the gas in said cells generates its characteristic light output at such a level that it masks the light output of said phosphor material. 